The invention relates to pixel front ends, primarily in readout devices for photodetector arrays and in particular to systems where the photodetectors require a wide bias voltage range and have high background currents, or both.
Many semiconductor photonic detectors (especially II-VI and III-V materials) operated in photoconductive mode must have voltage biases held relatively constant across their semiconductor junctions in order to maintain good linearity of a generated photocurrent, which is proportional to the incoming signal. Typically, this voltage bias will be applied to the detector junction by providing one common voltage to the detector substrate directly (called detector common) and one bias through a CMOS readout pixel. These CMOS readout pixels are typically implemented in an array format to create staring image sensors. There are several methods of applying this detector bias in CMOS circuitry. Active amplifiers with feedback are sometimes used in each pixel, such as that in resistive transimpedance amplifiers (RTIAs) or capacitive transimpedance amplifiers (CTIAs). Another popular (and perhaps older) method is through a passive amplifier made out of a single FET transistor, called a direct injection (DI) FET. Today, the direct injection approach is more common for small pixel or megapixel+ format image sensors due to the reduced area of a single transistor bias and the reduced power of a passive amplifier.
There are a number of problems with the typical bias methods provided above. The circuits are susceptible to detector shorts, which can damage the readout CMOS circuitry if the detector bias is high. None of the techniques are able to subtract unwanted dark current or background photocurrent from the incoming detector current without circuit modifications. Furthermore, many of the background subtracting modifications are only good for very high currents or on a global basis.
Many detector materials and/or types need a better solution. Examples such as Quantum Well Infrared Photodetectors (QWIPs), Strained Layer Superlattice (SLS), microfabricated bolometer arrays, or more conventional detectors operated at higher temperatures all have needs for a higher possible detector bias, larger possible detector current, dead short protection to protect the readout from individual bad detector elements, and some form of background subtracting charge skimming. Other detector materials may have similar needs, especially those containing photoconductive gain or very weak diode characteristics (low impedance reverse bias). High background current in particular is a driving force requiring many IR detector systems to be operated at cryogenic temperatures, leading to increased expense and complexity, lower reliability and portability, and limited operational life, so that possible operation at higher temperatures is clearly desirable. Thus it is the object of this invention to provide a pixel front end that in various embodiments can provide correction for high background currents, wide bias range and/or short protection, as well as other benefits which will become apparent in the following disclosure.